Discrepant cortisol values: comparison of radioimmunoassay and competitive protein binding technics

Radioimmunoassays are expected to yield values representing the antigen, i.e., insulin, growth, cortisol, etc. employed in producing the antibody. However, in a recent study of a synthetic ACTH which involved the measurement of cortisol levels (1), we found that above 20gamma%, the radioimmunoassay (RIA) yields cortisol values distinctly above those obtained with the competitive protein binding (CPB) technic. We now suggest a possible explanation for this discrepancy and have calculated a correction factor.